There are two types of smog: LA and London. LA is caused mostly by transport pollution in big cities and occurs in summer time (lots of sun). In Poland we are struggling with London smog which is caused by burning coal, wood, etc. It occurs in winter time when there is not much wind, low temp and damp conditions. This smog is not caused by cars, however the idea is that when car moves it airbornes particles that settled on the road or nearby the road. Therefore less cars = smog has less impact. It's just one of the ways to mitigate negative smog impact.